Apache Cassandra is a free and open-source, distributed, wide column store, NoSQL database management system designed to handle large amounts of data across many commodity servers, providing high availability with no single point of failure. Cassandra offers robust support for clusters spanning multiple datacenters, with asynchronous masterless replication allowing low latency operations for all clients.
There's a great talk by Patrick FcFadin titled Introduction to Apache Cassandra which I'd recommend if you want a good overview.
It's based on a paper of the same name: Cassandra - A Decentralized Structured Storage System 
by Ladis and Lakshman, 2009 which has a really great reference list.